collect annotation earlier for mapped_column present

collect annotation earlier for mapped_column present

Fixed issue with new dataclass mapping feature where arguments passed to
the dataclasses API could sometimes be mis-ordered when dealing with mixins
that override :func:`_orm.mapped_column` declarations, leading to
initializer problems.

the change made here is specific to the test case given which regards
mapped_column() most specifically.   cases that involve relationship()
etc. are not tested here, however mapped_column() is the only attribute
that's implicit without an instance given on the right side, and is also
most common for mixins.   not clear if there are more issues in this
area, however it appears that we need only adjust the order in which we
accommodate grabbing the annotations in order to affect how dataclasses
sees the class; that is, we have control over ``__annotations__`` here
so dont have to worry about ``cls.__dict__``.

Fixes: #8688
Change-Id: I808c86f23d73aa47cd910ae01c3e07093d469fdc